A patient interface, for example, a mask, is used to deliver positive pressure to a patient's airway to treat breathing disorders, such as sleep apnea. The patient interface is typically held against the patient's face in sealing contact by a head gear. Referring to FIG. 1, a model of a current patient interface and headgear system is shown. The patient 1 has a patient interface 2 in sealing contact with the face. A headgear may be provided to maintain the patient interface 2 in sealing contact with the face of the patient 1. The patient interface 2 may include a cushion, which is a deformable component. Forces 4 applied by straps of the headgear create a moment 6 perpendicular to the plane of the page.
As shown in FIG. 2, another current patient interface system is shown. The patient 1 has a patient interface 2 in sealing contact with the face. The patient interface 2 may be a mask that includes a forehead support that contacts the face of the patient 1 at a point 10 that may be a pivot point. Forces applied by the straps of the headgear create a moment 6 perpendicular to the plane of the page.
In the patient interface system shown in FIGS. 1 and 2, the moment 6 may make the patient interface 2 unstable on the face of the patient 1 under certain conditions. The patient interface systems shown in FIGS. 1 and 2 rely on the stiffness and deformation of the cushion of the patient interface 2 for correct location of the patient interface 2 on the face of the patient 1.
Referring to FIGS. 3a-3c, another current patient interface system is shown. The system shown in FIGS. 3a-3c is the Breeze™ manufactured by Puritan-Bennet. As shown in FIG. 3b, a headgear 11 maintains a patient interface 2 in contact with the face of the patient 1. The headgear 11 includes a pad 12a that contacts a top of the patient's head and a pad 12b that contacts the back of the patient's head. The two pads can be modeled as sliders and they identify a center of rotation 14. The reaction forces of the pads 12a, 12b can be reduced to a force vector 16 applied at the center of rotation 14 and a moment 18. Translation along the direction of the force vector 16 and rotation about the center 14 are still possible for the headgear 11. Forces 20 and 22 produced by the deformation of the patient interface 2 may balance. The headgear 11 also includes two symmetrical straps 11a (only one shown) on the sides of the head of the patient 1 that help give stability to the mask in respect to rotations around an axis laying on the plane of the page. As shown in FIGS. 3b and 3c, when the patient interface is removed from contact with the patient, as shown in FIG. 3c, the headgear 11 has a tendency to pop off the head of the patient when the reaction forces supplied by the patient interface are not present.
Referring to FIGS. 4a-4c, a patient interface system according to another known system is illustrated. The system illustrated in FIGS. 4a-4c is the Comfortlite2™ manufactured by Respironics Inc. A headgear 24 is provided to maintain contact between the patient interface 2 and the face of the patient 1. The headgear 24 includes straps 28 that apply forces that define a moment 30 around a center of rotation 29. The moment 30 tends to cause the headgear 24 to rotate about the points of contact 31, 32 with the patient's face. Any force applied to the headgear along the plane 34 (e.g. on the tube or conduit 26) can only be balanced by the reaction of the cushion of the patient interface 2. The forces given by deformation of the patient interface 2 are needed to keep the mask assembly stable on the head of the patient 1.